The invention relates to use of certain sulfur-containing silanes in sulfur-vulcanizable, silica-reinforced tire rubber compositions to provide a coupling agent with a high ratio of sulfur to silicon. In its more preferred aspects, the use of certain cyclic sulfur silanes in tire rubber compositions of this type produces tires exhibiting reduced rolling resistance and maintained or improved wet traction, modulus and abrasion resistance.
Rubber compositions for tire treads have traditionally employed carbon black as a principal reinforcing filler. However, due to recent increased demands for fuel efficiency and performance, the use of silica instead of carbon black has become more prevalent as a principal filler. In some cases, the use of silica in tire treads has been found to give lower rolling resistance without sacrificing abrasion resistance, modulus, or wet traction. However, in order for silica to be an effective reinforcing filler, an effective coupling agent is required. In this regard, see U.S. Pat. Nos. 3,451,458; 3,664,403; 3,768,537; 3,884,285; 3,938,574; 4,482,663; 4,519,430; 4,590,052; 5,066,721; 5,089,554; and 5,753,732 and British Patent No. 1,424,503. The coupling agents most often used in practice are polysulfide silanes of a type such as Silquest(copyright) A-1289 silane, which is a bis-3-(triethoxysilylpropyl)tetrasulfide silane, or Silquest(copyright) A-1589 silane, which is a bis-3-(triethoxysilylpropyl)disulfide silane. As has been established by a great deal of testing, not all coupling agents are effective in achieving the sometimes competing objectives of the present invention.
There is a need for the development of sulfur-vulcanizable, silica-reinforced tire rubber compositions that can employ silanes other than a limited group commercially available and known to give improvements in rolling resistance without having a detrimental impact on other properties. Norbornanylsulfursilanes, or trithiane silanes, are known for use in some applications as coupling agents, although not in tire rubber compositions for treads where their properties are unknown. See, for example, U.S. Pat. No. 4,100,172 to Mui, J. Y. P. and Kanner B., assigned to Union Carbide Corporation, New York, 1978. The previously published syntheses of trithiane silanes are disadvantageous and the advantages of the use of trithiane silane compounds and compositions as coupling agents in sulfur-vulcanizable, silica-reinforced tire rubber compositions has not been known. On the prior art process, see Shields, T. C. and Kurtz, A. N., Journal of the American Chemical Society, 1969, 91, 5415, and Bartlett, P. D. and Ghosh, T., Journal of Organic Chemistry, 1987, 52, 4937.
There remains a need for a process that enables the production of tires and parts such as treads for tires exhibiting reduced rolling resistance and road noise and maintained or improved wet traction, modulus and abrasion resistance, and this is provided by the invention through the use of sulfur silane coupling agent compounds and compositions in sulfur-vulcanizable, silica-reinforced tire rubber compositions.
It is an object of the invention to provide an improved process for preparing sulfur-vulcanizable, silica-reinforced tire rubber compositions and tires and tire parts, vulcanized and unvulcanized, made from these compositions.
It is another object of the invention to provide an improved process for preparing sulfur-vulcanizable, silica-reinforced tire rubber compositions and tires and tire parts, vulcanized and unvulcanized, made from these compositions, where tires made from the vulcanized rubber exhibit reduced rolling resistance.
It is another object of the invention to provide an improved process for preparing sulfur-vulcanizable, silica-reinforced tire rubber compositions and tires and tire parts, vulcanized and unvulcanized, made from these compositions, where tires made from the vulcanized rubber exhibit improved modulus.
It is another object of the invention to provide an improved process for preparing sulfur-vulcanizable, silica-reinforced tire rubber compositions and tires and tire parts, vulcanized and unvulcanized, made from these compositions, where tires made from the vulcanized rubber exhibit improved wet traction.
It is another object of the invention to provide an improved process for preparing sulfur-vulcanizable, silica-reinforced tire rubber compositions and tires and tire parts, vulcanized and unvulcanized, made from these compositions, where tires made from the vulcanized rubber exhibit reduced road noise.
It is another object of the invention to provide an improved process for preparing sulfur-vulcanizable, silica-reinforced tire rubber compositions and tires and tire parts, vulcanized and unvulcanized, made from these compositions, where tires made from the vulcanized rubber exhibit improved abrasion resistance.
It is still another object of the invention to provide an improved process for preparing sulfur-vulcanizable, silica-reinforced tire rubber compositions and tires and tire parts, vulcanized and unvulcanized, made from these compositions, where tires made from the vulcanized rubber exhibit reduced rolling resistance and road noise and maintained or improved modulus, wet traction and abrasion resistance.
These and other objects are achieved by the present invention, which provides a process for preparing sulfur-vulcanizable, silica-reinforced tire rubber compositions comprising:
preparing a blend of sulfur-vulcanizable rubber, silica reinforcing filler and a coupling agent comprising a sulfur-containing norbornanyl silicon compound of the structure
[Syxe2x80x94R]nxe2x80x94SiX4xe2x88x92n
wherein each X is chosen from monovalent hydrocarbon groups or hydrolyzable groups, including, but not limited to, alkoxy, halide or an oxygen which oxygen in turn is bonded to another silicon atom to form a siloxane; y is 1 to 5, when y is 1 the compound is an episulfide, and when y is 2 to 5 the sulfur atoms form a polysulfide wherein each sulfur atom is bonded to another sulfur atom and the terminal valences of the polysulfide are bonded to vicinal carbon atoms; n is 1, 2 or 3; R is a polyvalent polycycloaliphatic hydrocarbon radical.
Tires and tire parts, vulcanized and unvulcanized, made employing the above rubber compounds are also new.
Many of the preferred aspects of the invention are described below.
The following description will illustrate the use of preferred norbornanyl silicon compositions as coupling agents for sulfur-vulcanizable, silica-reinforced tire rubber compositions. Also described will be vulcanized and unvulcanized tire parts and tires employing these compositions. The invention is however, not limited to the specific compositions illustrated. Vulcanized tires made of these compositions and the preforms for preparing them are novel and exhibit improvement in one or more aspects of reduced rolling resistance, reduced road noise, improved modulus, improved wet traction and improved abrasion resistance.
This sulfur containing norbornanyl silicon compounds (cyclic sulfur silanes) used according to the invention are characterized by the following structure (Formula I):
[Syxe2x80x94R]nxe2x80x94SiX4xe2x88x92nxe2x80x83xe2x80x83(Formula I)
wherein each X is chosen from monovalent hydrocarbon groups or hydrolyzable groups, such as alkoxy, halide or an oxygen, which oxygen in turn is bonded to another silicon atom to form a siloxane; y is 1 to 5, when y is 1 the compound is an episulfide, and when y is 2 to 5 the sulfur atoms form a polysulfide wherein each sulfur atom is bonded to another sulfur atom and the terminal valences of the polysulfide are bonded to vicinal carbon atoms; n is 1, 2 or 3; R is a polyvalent polycycloaliphatic hydrocarbon radical including but not limited to the following: 
In the above structures, the valencies to the left would be connected to the sulfur so as to form a ring and the valency to the right would be connected to the silicon atom.
Each X may be the same or different. Among the hydrocarbon groups for X are alkyl groups, preferably lower alkyls including from one to four carbons. Among the hydrolyzable X groups are halides, such as chlorine, the lower alkoxy groups, preferably those containing from one to four carbons and siloxanes, such as trimethoxysiloxy and methyldiethoxysiloxy. Mixed alkoxy groups (e.g., one being methoxy and another being ethoxy are a potential embodiment. Dimers of the silane wherein the siloxy group is another thiane silane is contemplated as well, e.g., 1,3-bis[(ethyl)-2-(3,4,5-trithiatricyclo[5.2.1.02.6]decyl)]tetraethoxy disiloxane. The most preferred X groups are ethoxy and methoxy.
The group R can be derived from a variety of polycycloaliphatic compounds containing at least one reactive, strained double bond, such as those represented by Formulas II, III and IV, as follows: 
Where m=1 or 2, n=0 to 8, R1 is selected from the group of H, C1-C12 alkyl, or C2-C12 alkenyl group and R2 is a C2-C4 alkene.
Non-limiting examples of the above compounds of Formulas II, III and IV are preferably selected from the group g-k: 
The sulfur-substituted polycycloaliphatic compounds, as characterized by Formula I, can be produced by the reaction of sulfur from any suitable source with an aliphatically unsaturated precursor of the silicon compound according to Formula Va in the presence of an acid catalyst. Among the suitable sources of sulfur are any of those that are capable of providing sulfur for the reaction under conditions effective to produce the desired products, e.g., any of the allotropes of elemental sulfur, or a compound capable of donating free sulfur, such as, but not limited to, di-n-hexadecyl tetrasulfide, di-carboxymethyl tetrasulfide, di-methyl hexasulfide, bis-(dimethylthiocarbamyl)hexasulfide and bis(triethoxysilylpropyl)tetrasulfide. Preferably, the sulfur source is essentially free of water, more preferably completely free of water. The suitable unsaturated silicon compounds are defined as follows:
[R3]nxe2x80x94SiX4xe2x88x92nxe2x80x83xe2x80x83(Formula Va)
where R3 is a polycycloaliphatic group containing at least one reactive strained double bond and X is as defined for Formula I above. Exemplary of the R3 groups for the precursor compounds are groups 1 through q, as follows: 
The synthesis of compounds of Formula Va may be made by several routes, preferably the hydrosilation of a compound containing two double bonds such as, but not limited to, those compounds g to k, or from the Diels-Alder type reaction of a vinyl-containing silane with a suitable diene, such as but not limited to, cyclopentadiene, methylcyclopentadiene, dicyclopentadiene and methylcyclopentadiene dimer.
Suitable acid catalysts include common organic acids, such as acid chlorides like acetyl chloride, or inorganic acids, such as hydrochloric. However, the acid will preferably be a silicon compound of Formula Vb containing at least one Si-Cl bond, referred to herein as chlorosilanes, as follows:
[R3]nxe2x80x94SiXxcex54xe2x88x92nxe2x80x83xe2x80x83(Formula Vb)
The most preferred catalyst compounds are chlorosilanes of Formula Vb wherein at least one of Xxcex5 is occupied by a halogen, preferably chlorine, and the remainder are as defined for X, and R3 is a polycycloaliphatic group containing at least one reactive strained double bond as described above.
A typical example of the above reaction is shown in Equation I, as follows: 
The reaction may be performed without any solvent, but solvents can be employed where they add practical value to the process. The product can be distilled, but in general, this is not necessary, since the reaction proceeds cleanly and in high yield. Among the available solvents are aromatic and aliphatic hydrocarbons, alcohols, ketones and ethers. Among the aromatic hydrocarbons are xylene, toluene, and benzene. Among the aliphatic hydrocarbons are pentane, hexane, octane, isooctane, decane, cyclohexane and methylcyclohexane. Among the alcohols are methanol, ethanol, isopropanol, propanol, butanol, hexanol, octanol and t-butanol. The ketones are represented by methyl ethyl ketone, methyl isopropyl ketone and cyclohexanone. The ethers are represented by tetrahydrofuran, dioxane, dioxolane and glyme. Certain of the solvents with low boiling points might require performing the reaction under elevated pressure.
A general procedure for the synthesis of the sulfur containing norbornanyl silanes of Formula I by the above reaction will entail directly reacting with sulfur, a norbornenylsilane of Formula Va in the presence of a chlorosilane catalyst, in a molar ratio in the range of about 1000:1 to about 10:1. The molar ratio of the norbornenylsilane of Formula Va to moles of sulfur in the sulfur source will be about 1:3 to about 1:4. The reaction temperature is advantageously over 150xc2x0 C., preferably between 150xc2x0 C. and 200xc2x0 C. Preferably, the molar ratio of the norbornenylsilane of Formula Va to chlorosilane is in the range of about 1000:1 to about 100: 1. The process can be performed at any reaction pressure near ambient, preferably from 0.8-1.2 atmospheres. Again, the use of low-boiling solvents may require the use of elevated pressures. The exclusion of water from the reaction by the use of a drying agent in a drying tube, or by the use of inert gases such as nitrogen or argon is the preferred method of the present invention.
Non-limiting examples of compounds of Formula I are shown below: 
The cyclic sulfur silanes made in the above manner can be directly recovered from the reaction mixture, such as by decanting and/or filtering, if desired. The product will typically have a dark color if no treatment is undertaken, but color and impurities present will not affect its use in rubber compounds prepared by the invention. If desired, the color can be improved by vacuum distillation to produce a light yellow liquid. The products can be utilized in the form as recovered, without the use of additional purification for the new uses identified by the invention without detrimental effect caused by the unpurified product cyclic sulfur silanes or their component impurities.
The cyclic sulfur silanes silane compositions used according to the invention can be made in the manner discussed above and as exemplified in copending application entitled Synthesis Of Cyclic Sulfur Silanes (attorney""s docket no. 2062-SIL0046) filed in the names of K. J. Weller and L. Hwang on the same date as this application or in the manner as the products of U.S. Pat. No. 4,100,172, which is hereby incorporated by reference in its entirety. The full disclosure of the copending application is also incorporated by reference. A discovery and an advantage of the present invention is that the liquid reaction mixture of the process described in the noted copending application and above, can be employed to achieve high quality rubber compositions and products with no or little purification.
The cyclic sulfur silane compositions of Formula I are employed in natural and synthetic rubber compositions and blends of known and novel formulation, in amounts consistent with those previously employed for other silane coupling agents for the use in sulfur-vulcanizable, silica-reinforced tire rubber compositions. Exemplary of suitable amounts will be at least 2 parts per hundred parts rubber (PHR) and, preferably from about 4 to about 20 PHR, e.g., 6 to 12 PHR. The amount will also be related to the amount of silica employed, preferably the ratio by weight of silica to silane being in the range of from 4:1 to about 40: 1, more narrowly from about 6:1 to about 10:1. The ratio of the sulfur to the silicon in the compound of Formula I will also play a role in the amount of silane employed. Here, the amount of silane will vary in an inverse relation with the molar ratio of sulfur to silicon, with the higher amounts in the above ranges being required as the molar ratio is decreased from 3:1 to 1:1. The higher molar ratios of sulfur to silicon are preferred and a distinct advantage of this aspect of the invention. Another advantage of the invention, especially when employing these higher molar ratios of sulfur to silicon in the coupling agents, that the use of added sulfur can be eliminated for vulcanization in some applications. However, when used, molar ratios of added sulfur for vulcanization to sulfur in the silane can be varied within the range of from above 0 to about 100:1 or more, preferably from 2:1 to 20:1, more narrowly from 5:1 to 10:1. The required amount of silane will decrease as its relative sulfur content increases.
The Cyclic Sulfur Silanes may be used neat as a liquid or may be loaded onto a carrier so that they may be delivered as dry particulates to the rubber composition. Suitable carriers include porous polymers, high surface area silica and carbon black.
The Cyclic Sulfur Silanes may be used with other silanes in the formation of the rubber compound, including, but not limited to, silanes containing the following functional groups, mercapto, alkenyl, vinyl, acrylate, amino, methacrylate, isocyanato, epoxy, carbamato, polysulfide, thiocarabamato, thiocyanato, ureido, thiocarboxylate, and blocked mercptans. Preferably these silanes are dialkoxy or trialkoxy silanes. Specific silane groups include mercaptopropyl trialkoxy silane, bis(trialkoxysilylpropyl) disulfide, bis (trialkoxysilylpropyl) tetrasulfide, vinyl trialkoxysilane, oligomers of the foregoing, hydrolyzates of the foregoing and mixtures of the foregoing.
Exemplary of suitable rubber compositions are sulfur-vulcanizable synthetic rubber compositions. Representative examples of suitable rubber polymers include solution styrene-butadiene rubber (SSBR), styrene-butadiene rubber (SBR), natural rubber (NR), polybutadiene (BR), ethylene-propylene co- and ter-polymers (EP, EPDM), and acrylonitrile-butadiene rubber (NBR). The rubber composition preferably is comprised of at least one diene-based elastomer, or rubber. Suitable conjugated dienes are isoprene and 1,3-butadiene and suitable vinyl aromatic compounds are styrene and alpha methyl styrene. Thus, the rubber is a sulfur curable rubber. Such diene based elastomer, or rubber, may be selected, for example, from at least one of cis-1,4-polyisoprene rubber (natural and/or synthetic), and preferably natural rubber), emulsion polymerization prepared styrene/butadiene copolymer rubber, organic solution polymerization prepared styrene/butadiene rubber, 3,4-polyisoprene rubber, isoprene/butadiene rubber, styrene/isoprene/butadiene terpolymer rubber, cis-1,4-polybutadiene, medium vinyl polybutadiene rubber (35-50 percent vinyl), high vinyl polybutadiene rubber (50-75 percent vinyl), styrene/isoprene copolymers, emulsion polymerization prepared styrene/butadiene/acrylonitrile terpolymer rubber and butadiene/acrylonitrile copolymer rubber. An emulsion polymerization derived styrene/butadiene (E-SBR) might be used having a relatively conventional styrene content of 20 to 28 percent bound styrene or, for some applications, an E-SBR having a medium to relatively high bound styrene content, namely, a bound styrene content of 30 to 45 percent. Emulsion polymerization prepared styrene/butadiene/acrylonitrile terpolymer rubbers containing 2 to 40 weight percent bound acrylonitrile in the terpolymer are also contemplated as diene based rubbers for use in this invention.
See, any of U.S. Pat. No. 3,451,458, U.S. Pat. No. 5,110,969, U.S. Pat. No. 5,227,425 and U.S. Pat. No. 5,753,732, for examples of rubber compounds that can be improved with the invention with silica as a reinforcing agent. The disclosures of these patents are incorporated by reference in their entireties. Rubber compositions based on solution polymerized styrene butadiene are preferred.
The rubber compositions, in addition to at least one elastomer of synthetic or natural origin, will contain silica in amounts effective for reinforcing the rubber in its vulcanized state. The silica can be of the types known, for example described in U.S. Pat. No. 4,704,414, U.S. Pat. No. 5,227,425 and U.S. Pat. No. 5,753,732, and will be employed in amounts suitable for the reinforcing tires, especially those having low rolling resistance. The silica will be employed at a level of from about 5 to about 100 parts per hundred parts of rubber, preferably at least 30 parts silica. Higher or lesser amounts can be employed where appropriate. The xe2x80x9csilica-reinforced rubberxe2x80x9d compositions as can be improved by the invention do not exclude the presence of carbon black which will still be present as a preferred ingredient in minor amounts for purposes of coloring or as a carrier for additives, even including the silane coupling agents. In this latter regard, see U.S. Pat. No. 4,128,438 and U.S. Pat. No. 5,159,009, which are incorporated by reference in their entireties. The silica component will, however, be present in an amount greater than the carbon black in tires or tire rubber compounds.
Precipitated silicas are preferred fillers. The silica may be characterized by having a BET surface area, as measured using nitrogen gas, preferably in the range of 40 to 600, and more usually in a range of 50 to 300 m2/g. The BET method of measuring surface area is described in the Journal of the American Chemical Society, Volume 60, page 304 (1930). The silica typically may also be characterized by having a dibutylphthalate (DBP) absorption value in a range of 100 to 350, and more usually 150 to 300. Further, the silica, as well as the aforesaid alumina and aluminosilicate, may be expected to have a CTAB surface area in a range of 100 to 220. The CTAB surface area is the external surface area as evaluated by cetyl trimethylammonium bromide with a pH of 9. The method is described in ASTM D 3849.
Mercury porosity surface area is the specific surface area determined by mercury porosimetry. For such technique, mercury is penetrated into the pores of the sample after a thermal treatment to remove volatiles. Set up conditions may be suitably described as using a 100 mg sample; removing volatiles during 2 hours at 105xc2x0 C. and ambient atmospheric pressure; ambient to 2000 bars pressure measuring range. Such evaluation may be performed according to the method described in Winslow, Shapiro in ASTM bulletin, p.39 (1959) or according to DIN 66133. For such an evaluation, a CARLO-ERBA Porosimeter 2000 might be used. The average mercury porosity specific surface area for the silica should be in a range of 100 to 300 m2/g.
The rubber composition may be compounded by methods known in the rubber compounding art, such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, silicas, plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agents, and reinforcing materials such as, for example, carbon black. Other suitable filler materials can be used, such as metal oxides, e.g., titanium dioxide, aluminosilicate and alumina, siliceous materials including clays and talc, and carbon black. Depending on the intended use of the sulfur vulcanizable and sulfur vulcanized material (rubbers), the additives mentioned above are selected and commonly used in conventional amounts.
A Rubber Composition May Prepared By a Process Such as By:
(A) thermomechanically mixing, in at least one preparatory mixing step, to a temperature of 140xc2x0 C. to 200xc2x0 C., alternatively to 140xc2x0 C. to 190xc2x0 C., for a total mixing time of 2 to 20, alternatively 4 to 15, minutes for such mixing step(s)
(i) 100 parts by weight of at least one sulfur vulcanizable rubber selected from conjugated diene homopolymers and copolymers, and copolymers of at least one conjugated diene and aromatic vinyl compound, (ii) 5 to 100, preferably 25 to 80, phr (parts per hundred rubber) of particulate filler, wherein preferably the filler contains 1 to 85 weight percent carbon black (iii) 0.05 to 20 parts by weight filler of at least one cyclic sulfur silane;
(B) subsequently blending therewith, in a final thermomechanical mixing step at a temperature to 50xc2x0 C. to 130xc2x0 C. for a time sufficient to blend the rubber, preferably between 1 to 30 minutes, more preferably 1 to 3 minutes, a curing agent at 0 to 5 phr; and optionally
(C) curing said mixture at a temperature of 130 to 200xc2x0 C. for about 5 to 60 minutes.
An exemplary process for using silane coupling agents to manufacture silica containing tires is disclosed in PCT/US98/17391, which is incorporated herein by reference.
The rubber compositions of the invention are employed to form tire parts, such as treads and sidewalls in the normal fashion as conventional silica-reinforced, sulfur vulcanizable rubber compositions. Typically, a rubber composition is shaped into parts of rough shape and then placed within a tire mold wherein they are heated to effect vulcanization. There are some cases wherein one or more tire parts will be partially vulcanized prior to assembly into the tire. It is an advantage of the invention that it applies well to these conventional techniques, without the need to modify blending or production procedures.